In recent years, a development of an electronic device (a compound semiconductor device) is vigorous in which a GaN layer and an AlGaN layer are formed sequentially on a substrate composed of sapphire, SiC, GaN, Si, or the like, and the GaN layer is used as an electron transit layer. A bandgap of GaN is 3.4 eV, and it is larger than that of GaAs of 1.4 eV. Accordingly, an operation in high withstand voltage is expected for the compound semiconductor device.
A high-voltage operation is required for a base station amplifier of a cellular phone system, and a high withstand voltage is essential requirement thereof. At present, a value over 300 V is reported as a withstand voltage when current is turned off. There is a case when a change in on-resistance during operation called as a current collapse occurs in the above-stated compound semiconductor device. This phenomenon can be avoided by disposing a GaN protective layer on an AlGaN electron supply layer, and further forming an SiN protective film. The structure as stated above is described in, for example, Japanese Patent Application Laid-open No. 2002-359256.
FIG. 1 is a sectional view showing a structure of a conventional compound semiconductor device. An AlN layer 102, a GaN layer 103, an n-type AlGaN layer 104a, an n-type AlGaN layer 104b and an n-type GaN layer 105 are sequentially formed on an SiC substrate 101. A thickness of the AlN layer 102 is 150 μm. A total thickness of the n-type AlGaN layer 104b and the n-type GaN layer 105 is approximately 30 nm. Further, an SiN layer 107 is formed on the n-type GaN layer 105. Plural openings are formed at the SiN layer 107, and a source electrode 108a, a drain electrode 108b and a gate electrode 109 are evaporated in these openings.
But, this compound semiconductor device is a normally-on type. There is a case when a compound semiconductor device is used for automobile parts. Current continues to flow in the automobile parts when a car goes out of order resulting from a traffic accident and so on when the compound semiconductor device is the normally-on type. Accordingly, a demand for a normally-off type compound semiconductor device becomes high in recent times.
It is necessary to make a threshold voltage positive to realize the normally-off type compound semiconductor device. Accordingly, a recess gate structure is proposed. However, it is difficult to manufacture a compound semiconductor device in the recess gate structure with high accuracy. This is because a selective etching suitable for processing each semiconductor layer constituting the compound semiconductor device has not been found, and instead, a time-controlled dry etching is performed.
Besides, in the conventional compound semiconductor device shown in FIG. 1, a positive gate leakage current flows in a saturation region as input power is made large. This is because the gate electrode 109 and the n-type GaN layer 105 are in contact directly. It is difficult to continue to use in a saturation power state if the gate leak current as stated above flows, and it is hard to say that it is suitable for the base station amplifier of the cellular phone.
Accordingly, a structure in which an insulator layer is sandwiched between the n-type GaN layer 105 and the gate electrode 109 is proposed. However, when the insulator layer exists, the gate leak current decreases, but the threshold voltage becomes deep. Accordingly, it is considered that the threshold voltage cannot be made positive with this structure. Further, the withstand voltage decreases though the gate leak current is reduced. This is caused by an influence of a trap between the insulator layer and the n-type GaN layer 105.